This application claims the benefit of Korean Patent Application No. 2000-11884, filed on Mar. 9, 2000, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device. And more particularly, it relates to the LCD device having a cholesteric liquid crystal polarizer and cholesteric liquid crystal color filters.
2. Description of Related Art
In general, a liquid crystal display device uses the optical anisotropy and polarization properties of liquid crystal molecules. Because of their peculiar characteristics, the liquid crystal molecules have a definite orientational order in arrangement. The arrangement direction of the liquid crystal molecules can be controlled by an applied electric field. In other words, when electric fields are applied to liquid crystal molecules, the arrangement of the liquid crystal molecules changes. Since incident light is refracted according to the arrangement of the liquid crystal molecules, due to the optical anisotropy of liquid crystal molecules, image data can be displayed.
Of the different types of LCDs, an active matrix LCD (AM-LCD) (having thin film transistors and pixel electrodes are arranged in the form of a matrix) is the majority subject of research and development activity due to its high resolution and superiority in displaying moving images.
FIG. 1 is a cross-sectional view illustrating a related art liquid crystal display (LCD) panel. As shown in FIG. 1, the LCD panel 20 has lower and upper substrates 2 and 4 and an interposed liquid crystal layer 10. The lower substrate 2, which is referred to as an array substrate, has a TFT xe2x80x9cSxe2x80x9d as a switching element that changes the orientation of the liquid crystal molecules. A pixel electrode 14 applies a voltage to the liquid crystal layer 10 according to the state of the TFT xe2x80x9cSxe2x80x9d. The upper substrate 4 has a color filter 8 for implementing a color and a common electrode 12 on the color filter 8. The common electrode 12 serves as an electrode for applying a voltage to the liquid crystal layer 10. The pixel electrode 14 is arranged over a pixel portion xe2x80x9cPxe2x80x9d, of a display area. Further, to prevent leakage of the liquid crystal layer 10, the two substrates 2 and 4 are sealed using a sealant 6.
In the above-mentioned AM-LCD device, the data signal is applied to the pixel electrode 14 in accordance with the scanning signal of the gate electrode of the TFT xe2x80x9cSxe2x80x9d, i.e., the TFT xe2x80x9cSxe2x80x9d is turned ON. On the contrary, the data signal is not applied to the pixel electrode 14 when the TFT xe2x80x9cSxe2x80x9d is turned OFF.
The LCD device is a sort of light modulator and uses light from the backlight device (not shown). However, the LCD device is not efficient because the light generated from the backlight device has to pass through the several layers to display the color images. These several layers are a pair of linear polarizers, color filters, etc.
Especially, since the linear polarizer only transmits a linear component of the light, i.e., the linearly polarized light of the light from the backlight, the density of the light decreases. Namely, less than half of the light passes through the LCD panel such that the LCD device is not efficient at using the light from the backlight device. Therefore, the brightness is degraded. Moreover, the color filters that are used in the LCD device usually absorb the light except for the light having the intrinsic wavelength, and thus the optical density and the brightness is lowered.
For the purpose of solving the aforementioned problems, the light transmissivity of the color filters should be increased. And, for the purpose of raising the light transmissivity, the color purity should be lowered. However, there is a limitation upon decreasing the color purity.
Accordingly, a cholesteric liquid crystal color filter (referred to as CLC color filter hereinafter) and a cholesteric liquid crystal polarizer (referred to as CLC polarizer hereinafter) are researched and applied to the LCD device to improve the brightness. The CLC color filter has characteristics of both the liquid crystal and the color filter. Namely, the CLC color filter selectively reflects or transmits incident light, and selectively displays a certain color. Moreover, it is widely known that using the CLC color filter and CLC polarizer in the LCD device increases the efficiency of the light emitted from the backlight device.
FIG. 2 is a schematic sectional view of a related art LCD device having the CLC color filter and the CLC polarizer. As shown, the LCD device 25 has upper and lower substrates 40 and 30 corresponding to upper and lower substrates 4 and 2 of FIG. 1, and an interposed liquid crystal layer 38. CLC color filters 34 having red (R), green (G) and blue (B) colors are on the lower substrate 30. A black matrix 36 is positioned between the CLC color filters 34 and the lower substrate 30, in the boundary between the CLC color filters 34.
The CLC color filters 34 are color filters made of a cholesteric liquid crystal (CLC). They selectively reflect or transmit incident light. For example, if the molecular structure of the CLC is twisted in the right direction, the CLC reflect only right-handed circularly polarized light. Additionally, objects have an intrinsic wavelength. The color that an observer sees when looking at an object is the wavelength of the light reflected from or transmitted through the object. The wavelength range of visible light is from about 400 nm to 700 nm. Visible light can be broadly divided into red, green, and blue regions. The wavelength of the red light region is centered at about 660 nm, that of green is centered at about 530 nm, and that of blue is centered at about 470 nm. The pitch of the cholesteric liquid crystal is controllable and, therefore it is possible that a CLC color filter can selectively transmit light having the intrinsic wavelength of the color corresponding to a pixel. This enables a pixel to display red (R), green (G) or blue (B) with a high purity. To implement a precise color, a plurality of the CLC color filters can be selectively arranged. Therefore, a CLC color filter can display a selected color better than a conventional absorptive color filter.
Referring to FIG. 2, a backlight device 50 is located under the lower substrate 30. A CLC polarizer 32 is located under the lower substrate 30 and between the backlight device 50 and the lower substrate 30. The backlight device 50 generates artificial light that displays color images in accordance with the color filters 34. The CLC polarizer 32 is a polarizer made of a cholesteric liquid crystal. It reflects or transmits a left- or right-handed circularly polarized light. Therefore, the CLC polarizer 32 passes a much larger amount of light than the linear polarizer. The CLC polarizer 32 is used substantially for changing the phase of the light, i.e., converting the light into the left- or right circularly polarized light. A black matrix 36 that is made of an organic substance or a metallic material is formed on the lower substrate 30. The black matrix 36 is also arranged in the boundary between the CLC color filters 34 such that it divides the color filters 34 into the displaying areas.
Still referring to FIG. 2, a retardation film 42 and a linear polarizer 46 are formed in series on the upper substrate 40. The retardation film 42, which has a phase difference of xcex/4, respectively converts the circularly polarized light into the linearly polarized light, or the linearly polarized light into the circularly polarized light. The liquid crystal layer 38 is interposed between the upper substrate 40 and the lower substrate 30 and functions as an optical shutter for changing a direction or a phase of the light that is colored by the CLC color filters 34.
As mentioned above, since the CLC polarizer is adopted in the LCD device, the efficiency of the light from backlight device is raised; thereby the brightness of the LCD device increases. Moreover, since the CLC color filter is adopted in the LCD device instead of the absorptive color filter, the tint and color purity increase, compared to the LCD device that has the absorptive color filter.
FIG. 3 is a graph illustrating transmittance after light passes through each layers of a typical LCD device. The two polarizers have a transmittance of 45% and, the two substrates have a transmittance of 94%. The TFT array and the pixel electrode have a transmittance of 65%, and the color filter has a transmittance of 27%. Therefore, the typical LCD device has a transmittance of about 7.4% as seen in FIG. 3, which shows a transmittance (in relative brightness %) after light passes through each layer of the device.
In general, with respect to the LCD device, the pixel electrode is required to operate the liquid crystal layer, the TFT as a switching device, plural signal lines applying signals to the TFT, etc. Among them, the displaying portion is substantially an area where the pixel electrode is positioned. And the area except the displaying portion is covered with the black matrix.
If the aperture ratio of the LCD device is 40%, about 60% of the light from the backlight device is shielded by the black matrix. Therefore, for the purpose of the high brightness, the light from the backlight device should initially be much brighter, thereby electric power consumption by the backlight device increases.
In order to overcome the problem described above, the battery of the LCD device used for the portable computer has a high electric power consumption. So a relatively heavy battery is needed to supply a sufficient power to the backlight device. However, this has a problem that the battery cannot be used for a lengthy period of time. Moreover, when using an organic substance as a black matrix, the organic black matrix absorbs the light, and thus the light efficiency is reduced. When using a metallic material as a black matrix, the light is reflected by the metallic black matrix and then reflected again by the CLC polarizer, and thus the contrast of the LCD device is deteriorated.
Accordingly, the present invention is directed to a liquid crystal display device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
To overcome the problems described above, an object of the present invention is to provide a liquid crystal display device having cholesteric liquid crystal as a black matrix.
Another aspect of the present invention is to provide a, liquid crystal display device that increases efficiency in the use of the light from the backlight device.
To achieve these and other advantages and in accordance with the purposes of the present invention, as embodied and broadly described, there is provided an LCD device that includes first and second substrates facing and spaced; apart from each other; a liquid crystal layer interposed between the first and second substrates; (optionally) a retardation film formed on the first substrate and on the opposite side of the liquid crystal layer; (optionally) a second polarizer formed on the retardation film; a first polarizer formed under the second substrate and on the opposite side of the liquid crystal layer, the first polarizer made of cholesteric liquid crystal; color filters formed on the second substrate and facing the first substrate, wherein each color filter has one of a plurality of primary colors, e.g., red, green and blue; a black matrix formed in the boundaries between the color filters, wherein the black matrix is on the second substrate, and wherein the black matrix is made of the same material as the second polarizer; and a backlight device arranged under the second substrate.
The color filters are beneficially made of the cholesteric liquid crystal, and the black matrix reflects the light that passes through the first polarizer. The first polarizer transmits the light that is reflected by the black matrix.
To achieve the above aspects, in another aspect, the present invention provides an LCD device. That device includes first and second substrates facing each other and spaced apart from each other; a liquid crystal layer interposed between the first and second substrates; (optionally) a retardation film formed on the first substrate and on the opposite side of the liquid crystal layer; (optionally) a second polarizer formed on the retardation film; a first polarizer formed on the second substrate and facing the liquid crystal layer, the first polarizer made of cholesteric liquid crystal; color filters formed on the first polarizer and facing the first substrate, wherein each color filter has one of a plurality of primary colors, e.g., red, green and blue; a black matrix formed in the boundaries between the color filters, wherein the black matrix is on the second polarizer, and wherein the black matrix is made of the same material as the second polarizer; and a backlight device arranged under the second substrate.
To achieve the above aspects, in another aspect, the present invention provides an LCD device. That device includes a backlight device irradiating light; a polarizer converting the light from the backlight device into a first circularly polarized light, the polarizer made of cholesteric liquid crystal; color filters coloring the first circularly polarized light one of a plurality of primary colors, e.g., red, green and blue; a black matrix formed in the boundary between the color filters, wherein the first circularly polarized light is reflected by the black matrix and then passes through the polarizer; and a reflective plate formed in the backlight device; wherein the first circularly polarized light reflected by the black matrix and passing through the polarizer is converted as it reflected by the reflective plate of the backlight device into the second circularly polarized light; wherein the second circularly polarized light is reflected by the polarizer and reaches the reflective plate; wherein the second circularly polarized light reflected by the polarizer is converted into the first circularly polarized light, and then the first circularly polarized light passes through the polarizer and through the color filters.
The black matrix is beneficially made of the same material as the polarizer, and the first circularly polarized light is a left-handed circularly polarized light or a right-handed circularly polarized light.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate one embodiment of the invention and together with the description serve to explain the principles of the invention.